Titanium dioxide generally used as white pigments comprises usually particulate particles, and there are also known titanium dioxide particles having special particle shapes such as columnar and acicular shapes. These titanium dioxides which have a major-axis length larger than minor-axis length and an aspect ratio (major-axis length/minor-axis length), and are used, utilizing this anisotropic shape, as bases for electro-conductive materials, reinforcing materials, catalysts, catalyst carriers, special pigments, etc.
As processes for producing such titanium dioxides having anisotropic shape, there are known, for example, a process of mixing a usual rutile titanium dioxide with sodium chloride and an oxyphosphorus compound, and heating and firing the mixture to obtain acicular particles having a minor-axis length of 0.01-0.5 μm and an aspect ratio of 3-50 (Patent Document 1), a process of heating and firing a mixture comprising a titanium source, an alkali metal source and an oxyphosphorus compound in the presence of acicular titanium dioxide nucleus crystals to obtain acicular particles which have a weight-average minor-axis length of 0.05-0.8 μm and a weight-average major-axis length of 3-7 μm, and in which 70% by weight or more of the particles have a major-axis length of 2 μm or longer (Patent Document 2), etc.
Patent Document 1: JP-B-47-44974
Patent Document 2: JP-A-1-286924